1. Technical Field
Embodiments of the present invention relate generally to cryosurgical instruments such as cryoprobes and, more particularly, to phase separation of cryogen in cryosurgical instruments for heat exchanging advantage.
2. Description of related art
It is known to employ coiled heat exchangers in cryosurgical instruments with cryogen supplied as a pressurized gas. In such heat exchangers, the expansion of the gas causes it to change its temperature. As explained in U.S. Pat. No. 6,706,037, for example, this phenomenon is referred to as the “Joule-Thomson effect”, thermodynamically known as adiabatic expansion.
It is also known, in coiled heat exchangers, to use gases that cool upon expansion, such as Argon or Nitrogen, for cooling, and gases that heat as a result of the expansion, such as Helium, for heating. In these heat exchangers, the return gases are commonly used to recycle the thermal energy (i.e., the return cooling gas not only cools the outer surface of the cryosurgical instrument but also the inlet pressurized gas). The coil shape increases the contact area between cool fluids to hot fluids. The heat transfer between these fluids is proportional to the contact surface between them. Thus, the coil serves as an advantageous solution for such heat regeneration. See, for example, U.S. Pat. No. 6,706,037 (at column 15 lines 40-58, and column 16 lines 65-68). Also, as discussed in U.S. Pat. Nos. 3,800,552 and 5,522,870, lowering the inlet temperature may liquefy the expanded gas.
Still further, approaches to enhancing the cooling and heating regeneration are known. For example, U.S. Pat. No. 5,800,487 discusses enhancing the cooling and heating regeneration by supplying the inlet pressurized gas tube with fins to increase the area of the heat exchanging.
If the cryogen is supplied as a liquid or a two-phase (gas/liquid) fluid, cooling methods use the energy required for the change of phase of the inlet cryogen. In this case, the temperature of the inlet fluid and the return fluid may be the same or close in value, and would minimize, or annul heat exchange between the inlet and outlet fluids, due to a small differential temperature. The application of a coil serves as a separator of the phases in a two-phase flow. This is because the liquid phase has greater specific gravity, which tends to cause the liquid phase toward the outer (greater) diameter. The greater diameter of the coil is close to the external surface and, as a result, the boiling or change of phase of the liquid occurs close to the external surface. This arrangement utilizes the physical shape of the coil as a centrifugal force generator to separate the heavier liquid phase from the lighter gaseous phase, forcing (urging) the liquid towards the outer surface of the coil.
When the cryogens, either in gaseous form or two-phase form, enter in a straight inlet tube, or lumen, flow directing elements (such as those described in U.S. Pat. No. 5,800,487) or baffles (such as those described in U.S. Patent Publication No. 2009/0163902 A1, U.S. Patent Publication No. 2007/0149957 A1, U.S. Patent Publication No. 2007/0149959 A1, and U.S. Patent Publication No. 2009/0182320 A1) have been used.